Referring to FIG. 1, an initiator circuit for a flourescent tube is shown in schematic form. The terminal A and B are connected to an AC power source at a normal voltage of 110 volts. Clockwise from terminal B, the right cathode filament 11b of flourescent tube 10, conventional glow starter 14, power switch 13, the left cathode filament 11a of flourescent tube 10, and ballast 12 are all connected in series between terminals A and B. Capacitor 15 is connected in parallel with the terminals of conventional glow starter 14.
The conventional glow starter 14 is normally enclosed in a plastic, cylindrical module whose internal structure is shown in FIG. 2. A hermetically sealed glass bulb 142, filled with a mixture of argon and neon gas, is enclosed by translucent plastic enclosure 141. Contact plate 143 and bimetallic strip 144 made of two metallic alloys of dissimilar coefficients of thermal expansion to facilitate expansion and contraction of the strip with changes in temperature, are attached to electrically conductive support wires 145a and 145b, respectively, which in turn extend through the base of glass bulb 142 to attach with respective terminals 146a and 146b disposed on the bottom of the glow starter 14.
In operation, the power switch 13 of FIG. 1 is first closed which impresses an AC voltage across the terminals 146a and 146b of the glow starter 14 with little voltage drop from the other components.
The voltage is sufficient to cause ionization of the gas between contact plate 143 and bimetallic strip 144, causing a glow discharge between the two. The glow discharge heats the bimetallic strip 144 causing it to expand until it makes contact with contact plate 143, at which point the glow discharge is extinguished and the circuit is closed. Current flowing through cathode filaments 11a and 11b heats them to a temperature where thermionic emission occurs initiating ionization of the gas within the flourescent tube 10 and evaporating the mercury within it.
With the glow discharge extinguished, the bimetallic strip 144 cools and contracts away from contact plate 143, breaking the circuit and returning to its original position. Once broken, a large inductive back EMF is generated by ballast 12 which sparks an ion current between cathode filaments 11a and 11b to initiate normal operation of flourescent tube 10. The voltage is then effectively shunted across the flourescent tube 10, leaving insufficient voltage between the terminals 146a and 146b to cause further ionization in the glow starter 14.
The capacitor 15 in parallel with the glow starter 14 serves to aid in stabilizing the fluctuating currents concomitant with the tube's operation as does the ballast.
A major drawback of a conventional glow starter is that the bimetallic strip 144 has a relatively long time constant, i.e., it requires a relatively long period of time for the bimetallic strip 144 to heat sufficiently for closure with contact plate 143. This is due to the relative thickness of bimetallic strip 144 and its concomitant high heat capacity required for the bimetallic strip 144 to remain in contact with the contact plate 143 for a duration of time to sufficiently heat the cathode filaments 11a and 11b.
In the improved glow starter of the present invention, much thinner bimetallic strips are used with shorter time constants, a different bimetallic strip being used for the successive functions of making and breaking the initiator circuit.
This not only leads to shorter turn-on times for flourescent tubes but aids in extending the service life of the tubes by avoiding over heating of the cathode filaments.